The present invention relates to a focusing apparatus for a video camera or the like, and more particularly to a focusing apparatus suitable for the attainment of focusing through manual operation.
The basic structure of a lens system of a video camera or the like includes an object lens group 17, a variator lens group 18, a compensator lens group 19, a rear lens or master lens group 21, an image pickup element 22 and a diaphragm device 20, as shown by a schematic view of FIG. 1. The object lens group 17 has a function of taking the focus of each desired object at any distance. The variator lens group 18 has a variable power or multiplying function associated with zooming. The compensator lens group 19 is movable interlocking with zooming to perform a compensation function of preventing the desired object from being taken out of focus during zooming. The master lens group 21 has an imaging function of forming an image of the object onto the image pickup element 22.
In a zoom lens used in such a video camera, focusing or the adjustment of focus is usually effected by the movement of the object lens group. A so-called automatic focusing apparatus in which the adjustment of focus is automatically made, is based on an infrared ranging system or TTL projection system, as described by National Technical Report, Vol. 31, No. 6, pp. 65-67, December 1985 (hereinafter referred to as prior art (A)). The prior art (A) includes no particular description of the case where focusing is performed through manual operation. However, the prior art (A) discloses a structure in which an automatic/manual change-over switch is usually provided to allow a change-over between automatic focusing and manual focusing, as required, more especially, in a structure in which a manual focus adjustment is made in such a manner that a focus ring 16 mounted on a lens tube or sleeve 100 and interlocking with the object lens 17 as shown in FIG. 2 is rotationally moved through directly hand-worked operation, thereby performing a fine focus adjustment.
In recent years, compact articles or goods which are workable (or easy to deal with), of low cost and handy to carry are greatly required with rapid demands for cameras combined with VTR's (such as movies). In order to cope with such requirements, there has been reported an article as disclosed by "Hitachi's Camera Combined with VTR; Mastax Movie C30" on page 17 of the "Hitachi" for April 1987 (hereinafter referred to as prior art (B)). In a structure disclosed by the prior art (B), the entire lens system is incorporated into a movie casing.
With the structure disclosed by the prior art (B), it is not possible to take the focus by the rotational movement of a focus ring for lens through directly hand-worked operation as is done by the prior art (A). Therefore, the prior art (B) employs a manual focusing mechanism called a power focusing mechanism, as is also suggested by the prior art (A), in order to satisfy the above-mentioned requirements for compact and workable articles. A concrete example of the power focusing mechanism is not ,disclosed by the prior art (A) but can be realized as shown in FIG. 3.
FIG. 3 shows a control circuit for a zoom lens generally used in a video camera. In the figure, reference symbols A and B designate terminals applied with control signals for automatic focusing, numeral 101 designates an automatic/manual change-over switch, and symbols SW.sub.1 and SW.sub.2 designate button switches for power focusing. In each of the switches SW.sub.1 and SW.sub.2, opposite contacts thereof are connected with each other during only a time when a button of that switch is depressed. Numeral 102 designates a focusing motor, and numeral 103 designates a motor driving circuit which includes transistors Q.sub.1 to Q.sub.7 and resistors R.sub.1 to R.sub.6. In a state in which the automatic/manual change-over switch 101 is turned to the manual side (or switch open side) and the button switch SW.sub.1 is depressed, the transistors Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.8 are turned off while the transistors Q.sub.4, Q.sub.5, Q.sub.6 and Q.sub.7 are turned on so that a motor current flows in a direction indicated by arrow a along a path of power source (V.sub.CC).paren close-st.transistor Q.sub.4 .paren close-st. motor 102.paren close-st. transistor Q.sub.7 .paren close-st. earth (G), thereby rotating the motor 102 (for example, in a positive or forward direction). On the other hand, in a state in which the button switch SW.sub.2 is depressed, the transistors Q.sub.4, Q.sub.5, Q.sub.6 and Q.sub.7 are turned off while the transistors Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.8 are turned on so that a motor current flows in a direction indicated by arrow b along a path of power source (V.sub.CC).paren close-st. transistor Q.sub.8 .paren close-st. motor 102.paren close-st. transistor Q.sub.3 .paren close-st.earth (G), thereby rotating the motor 102 in a negative or reverse direction. In a state in which the button switches SW.sub.1 and SW.sub.2 are not depressed, the transistors Q.sub.1, Q.sub.2, Q.sub.3, Q.sub.5, Q.sub.6 and Q.sub.7 are turned on and the transistors Q.sub.4 and Q.sub.8 are turned off so that opposite ends of the motor 102 are grounded through the transistors Q.sub.3 and Q.sub.7 and hence no motor current flows, thereby keeping the motor 102 in a stopped condition.
The repetition of the forward rotation, reverse rotation and stoppage of the motor through the above-mentioned operation of the button switches allows focusing by fine adjustment, that is, power focusing.
The foregoing has been explained in conjunction with the case where focusing is effected by means of the object lens. Also, a so-called rear focusing system (master lens focusing system) is known in which focusing is effected by moving the master lens. In such a case, the coupling of the master lens with an interlocking relation with the focus ring is difficult for structural reasons and hence a power focusing device is an indispensable mechanism.
In the structure disclosed by the prior art (A), since the focus ring for lens is rotated through directly hand-worked operation, focusing can be attained rapidly and with high precision. However, it is difficult for structural reasons to make the article compact and the structure is hard to deal with. On the other hand, the structure disclosed by the prior art (B) is excellent in compactness of the article. However, in the case of the so-called power focusing device by which focusing is made through control for motor or lens movement/stoppage by the operation of button switches, the operation itself is conveniently simplified but a delicate button operation is required for taking the focus with high or excellent precision. Also, due to the influence of inertia of the lens and/or motor, etc., it is difficult to accurately stop the lens at a desired position. Further, when the speed rotational movement of the motor or lens is lowered, for example, by decreasing the motor current, a time required for focus adjustment becomes inconveniently long though the requirement for delicate button operation can be moderated and the influence of inertia can be reduced. Furthermore, even if a rear focusing system is employed, focusing must be performed by use of the power focusing device and hence it is difficult like the prior art (B) to accurately stop the lens at a desired position.
U.S. application Ser. No. 233,011 filed on Jul.18, 1988 by Murakami et al and entitled "Focusing Apparatus of Video Camera or the Like" has proposed an apparatus in which focusing to a position corresponding to the position of a contact or slider of a variable resistor manually operated is made by means of a feedback control system. If this apparatus in which the focus is taken depending on the position of the slider of the variable resistor is used in, for example, a system in which a change-over between automatic focusing and manual focusing is possible, the change-over from automatic focusing to manual focusing causes a difference between a position at which focusing has been attained through automatic operation and a focal position which corresponds to the present position of the slider of the variable resistor, so that the in-focus position of the camera is inconveniently feedback-controlled toward a focal position corresponding to the position of the slider.